The present invention relates to a power module which connects leads through ultrasonic bonding, and a manufacturing method of the power module.
A generally employed power module is configured by forming a wiring pattern formed of copper or the like on an insulated substrate formed of ceramics, and allowing a plurality of semiconductor devices to be mounted and stored in the module. For the thus configured power module, a wire bonding is used for connecting the electrode of the semiconductor device and the wiring pattern, the wiring patterns, and the wiring pattern and an external connection lead. The method for bonding a plurality of wires in parallel so as to apply high current has been disclosed in Japanese Patent Application Laid-Open Publication No. 2007-67342.
The recently introduced method uses a plate-like lead capable of reducing a region occupied by connection to be smaller than the region upon wire bonding in compliance with demands of a compact power module. The method of connecting the plate-like lead using ultrasonic waves has been introduced. For ultrasonic bonding (ultrasonic connection) of the plate-like lead is disclosed in, for example, Japanese Patent Application Laid-Open Publication Nos. 2010-40714, 2010-82668, 2010-212645, and 2011-61105.
In the method of performing ultrasonic bonding of the plate-like lead, the lead is placed on the wiring pattern, and presses the lead from above by applying a predetermined load using a bonding tool for horizontal ultrasonic vibration at a predetermined frequency. This makes it possible to allow bonding by removing stain and oxidized film on the metal surface under the load and ultrasonic vibration (amplitude).
In compliance with demand of high current application, the region of connection between the wiring pattern and the lead has been increased. The larger connection region demands increase in the pressing load and amplitude of the ultrasonic vibration for the purpose of achieving improved bonding, which may in turn have possibilities of damage to the insulated substrate.
Japanese Patent Application Laid-Open Publication No. 2010-40714 discloses the art of preventing damage to the insulated substrate. More specifically, each of the bonding member and bonded member contains copper as a main component, and a tin layer which contains tin as a main component is interposed between bonded surfaces of the bonding member and the bonded member. In the aforementioned structure, the bonding member is subjected to the ultrasonic vibration at a room temperature in press contact with the bonded member while having the tin layer interposed between those members.
Japanese Patent. Application Laid-Open Publication No. 2010-82668 discloses the structure for alleviating the force upon the ultrasonic bonding by providing the resin layer between an end portion of a terminal and the wiring.
Japanese Patent Application Laid-Open Publication No. 2010-212645 discloses provision of a thin buffer metal layer that is soft or exhibits high slidability between an end of the terminal and an electrode pad for the purpose of preventing abrasion and crack in the pad caused by the end of the terminal during ultrasonic bonding.
Japanese Patent Application Laid-Open Publication No. 2011-61105 discloses the coating layer formed on the pad, which is harder than the pad and the lead terminal. The coating layer will be destroyed during the ultrasonic bonding so that the lead terminal and the pad are bonded through plastic flow.
With the method disclosed in Japanese Patent Application Laid-Open Publication No. 2010-40714 for ultrasonic bonding of the plate-like lead, the material for forming the bonding member (lead) and the bonded member (substrate wiring pattern) is limited to copper. Combination with the other material for forming those members is not considered.
The process having the tin layer formed between the bonding surfaces for bonding is employed. The tin layer partially remains although most part of such tin layer is dispersed in the copper. There may be a case that the power module is brought into a high temperature state depending on the subsequent manufacturing inspection process, usage environment, or service condition. The power module in the high temperature state may soften the partially remained tin layer on the bonding surface to deteriorate strength of the interfacial bonded surface.
With the method disclosed in Japanese Patent Application Laid-Open Publication No. 2010-82668, the resin layer provided between the end of the terminal and the wiring is separated through the ultrasonic bonding, and further spread between the terminal and the wiring. This may reduce the intermetallic contact region between the terminal and the wiring, leading to increase in the electric resistance of the connection portion.
With the method disclosed in Japanese Patent Application Laid-Open Publication No. 2010-212645, the buffer metal layer may prevent abrasion and crack in the pad in the initial stage of the ultrasonic bonding. However, there is no consideration described with respect to prevention of the abrasion and crack in the pad at the portion with no buffer metal layer when the contact region between the pad and the electrode becomes large. With the method disclosed in Japanese Patent
Application Laid-Open Publication No. 2011-61105, the lead terminal and the pad are connected through plastic flow at the portion where the coating layer formed on the pad, which is harder than the pad and the lead terminal is cracked. When the contact region between the pad and the electrode becomes large, the coating layer that remains between the pad and the lead terminal prevents metal bonding between the pad and the lead terminal. This may increase connection resistance between the pad and the lead terminal, and may further deteriorate connection reliability.